The present disclosure is directed to operation of a furnace having a plurality of burners and containing a plurality of process tubes. In particular, the present disclosure is directed to operation of a furnace having a plurality of burners and containing a plurality of process tubes with improved efficiency.
Steam hydrocarbon (steam methane) reformers are furnaces containing numerous process tubes (including configurations with more than 400 reactor tubes), each tube containing catalyst (for example, a reforming catalyst) for transporting a process fluid (for example, steam and a hydrocarbon). The furnace, for example, can include vertically extending process tubes positioned in parallel rows with about 30 to 60 tubes in each row. The distance between two rows of tubes is about 2 to 3 meters. The tubes can extend vertically about 12 meters and have an outer diameter of 100 to 150 millimeters. The tubes can be positioned in the row with a center-to-center spacing of 250 to 500 mm. About 10 to 20 burners can be positioned between each set of two rows of tubes. A total of eight or more rows of tubes and nine or more rows of burners can be included in a furnace.
Generally, energy efficiency for industrial processes is becoming more important. For many processes, such as hydrogen production, the efficiency of the process is related to the ability to monitor and regulate certain temperatures. Measuring temperatures of reformer tubes in a reformer furnace can present several challenges. For example, when measuring temperatures at specific locations on the reformer tubes, inconsistency in taking the measurements at the specific location of the reformer tube can result in measurements with greater uncertainty. More precise monitoring of the temperature at the specific location on the reformer tube can permit improved energy efficiency by permitting more accurate data to be used for process control.
Furnace tube temperatures may vary along the length. The tubes may get hotter in the direction of the process flow as the process stream picks up heat from the furnace. Process tubes may cool due to endothermic reaction even as heat is added from the furnace. This axial variation is part of the process. Traditional methods of furnace control require a measure of temperature. This can be a tube wall temperature, a process gas temperature or a combustion gas temperature (or some combination). In traditional methods of furnace control, the overall flow of fuel (or in some cases oxidant or inerts) is adjusted to control the temperature as described in U.S. Pat. Publ. US2007/0104641. Adjustments also may be made to control the axial temperature profile.
Tube temperature may also vary from one tube to another. If there is axial variation it is necessary to compare tubes at the same axial position to determine the tube-to-tube variability. There may be operational benefits to reducing the tube-to-tube variability or to controlling the variability. The methods described here are intended to address the issue of tube-to-tube variability or furnace balance. This is done in addition to the traditional control methods which adjust the overall flow of fuel (or other stream) to control temperature.
Regulating temperatures in a furnace having process tubes and a plurality of burners for heating the process tubes can also present several challenges. The complex interaction of flame heating from the plurality of burners coupled with the uncertainty of temperature measurements has heretofore limited efficiency gains.
One way to improve the efficiency of a reformer furnace is to maintain a uniformity of temperature among the process tubes at a predetermined elevation in the furnace. Thus, the measuring or monitoring of the temperature of each of the process tubes at a predetermined location or elevation can be needed to obtain the desired efficiency improvement. In addition, the process tubes of a furnace can be under very high internal pressures (up to about 50 atmospheres) and at very high temperatures (up to about 950° C.). Thus, a slight change in temperature can reduce the operational life of a reactor tube. For example, operating at about 10° C. above the design temperature for the tube can reduce the operational life of the tube by as much as one half. The cost of repairing and/or replacing the tubes can be high due to the use of special alloys in the tubes that are needed to permit the tubes to survive the operational conditions of the furnace. As such, furnace operators also measure/monitor the tube temperatures to avoid exceeding the tube design temperature in addition to trying to obtain efficiency improvements.
Industry desires to operate furnaces containing process tubes with a uniformity of temperature among the process tubes at a predetermined elevation in the furnace.